Cation exchange membrane and use thereof in the electrolysis of sodium chloride

ABSTRACT

Cation exchange membranes characterized by carboxylic or carboxylic and sulfonic acid groups pendant from a fluorocarbon polymer are utilized for the electrolysis of aqueous sodium chloride.

RELATED APPLICATIONS

This application is a divisional of copending application Ser. No.836,417, filed on Sept. 26, 1977, which is a continuation-in-part ofapplication Ser. No. 745,196, filed on Nov. 26, 1976, now abandoned,which is, in turn, a continuation of application Ser. No. 556,288 filedon Mar. 7, 1975, now abandoned.

BACKGROUND OF THE INVENTION

A large proportion of chlorine and caustic produced throughout the worldis manufactured in diaphragm type electrolytic cells wherein the opposedanode and cathode are separated by a fluid permeable diaphragm which maybe of asbestos, a polymer film or a polymer film supported on asbestos.The diaphragm defines separate anolyte and catholyte compartments.Chlorine is produced in the former, aqueous sodium hydroxide in thelatter.

In operation, brine, preferably saturated, is fed to the anolyte. Thebrine passes through the diaphragm into the catholyte compartment wherean aqueous sodium hydroxide solution of about 11 to 18 percent isproduced. This solution is contaminated by sodium chloride which must beseparated. After separation, the caustic solution is concentrated toproduce the commercial product.

The permionic exchange membrane type electrolytic cell has been utilizedas an improvement over the diaphragm type. The permionic membranereplaces the diaphragm, and is characteristically different from itsince, in contrast to the diaphragm, it is substantially impervious towater and to sodium chloride. The exchange membrane selected for theproduction of chlorine and caustic is usually a cation exchange membranewhich permits the passage of sodium ions into the catholyte, butprevents back-migration of OH ions into the anolyte. As a result,relatively pure caustic substantially free of sodium chloride isproduced in the catholyte, and high grade chlorine is produced at theanode.

A number of cation exchange membranes are known.

U.S. Pat. Nos. 3,887,499 and 3,657,104 describe permselective cationexchange membranes comprising a hydrocarbon polymer backbone withpendant carboxylic and sulfonic groups.

U.S. Pat. No. 3,878,072 describes cation exchange membranes which arehydrolyzed copolymers of a perfluororinated hydrocarbon and either afluorosulfonated perfluorovinyl ether or a sulfostyrenatedperfluorinated ethylene propylene polymer. In either event, thecharacteristic feature of the membrane is the sulfonyl group as the onlyfunctional group.

U.S. Pat. No. 3,853,721 which issued on Dec. 10, 1974 describes asbestosdiaphragms containing from about 0.01 to 22 weight percent, based on theweight of the diaphragm, of an ion exchange resin which is afluorocarbon polymer characterized by the presence of the followinggroups:

sulfonic--SO₃ H

fluoromethylene sulfonic--CF₂ SO₃ H

benzene sulfonic--φSO₃ H

chloromethylene sulfonic--CC1₂ SO₃ H

carboxylic--COOH

phosphoric--PO₃ H₂

phosphorus--PO₂ H₂

phenolic--φOH

A characteristic feature of these diaphragms is that they are gas andelectrolyte permeable in contrast to permionic membranes, such as thecationic ion exchange membranes of this invention which aresubstantially impermeable to electrolytes, but permit the passage ofions. Another feature, according to the patent, is that they have aresistance voltage drop across the diaphragm of as much as 0.2 to 0.3volt less than an untreated asbestos diaphragm of the same thickness.

A problem with the use of diaphragm electrolysis, as pointed out above,is that the permeability of the diaphragm to sodium chloride results inaccumulation of this material in the catholyte. This concentration maybe as high as 17%. When attempts are made to produce concentratedsolutions of sodium hydroxide in the catholyte by evaporation of water,it is necessary to use an expensive apparatus as well as a large amountof energy.

U.S. Pat. No. 3,301,893 discloses certain fluorocarbon ethers containingboth carboxyl and sulfonyl groups. The products of this patent, however,are of such low molecular weight that they cannot be utilized for thepreparation of cation exchange membranes.

THE INVENTION

It has now been discovered that many of the difficulties of the priorart can be minimized or completely avoided by the utilization ofpermselective cationic ion exchange membranes in which carboxylic acidgroups or both carboxylic and sulfonic acid groups are pendant from afluorocarbon polymer.

Membranes of this type manifest a number of significant advantages.Those characterized by the presence of carboxyl groups manifest:

1. Decreased back-migration of hydroxyl ions.

2. Increased current efficiency at high current density even when theconcentration of sodium hydroxide in the catholyte is high.

3. Increased purity of the sodium hydroxide solution produced in thecathode because of the resistance of the membrane to permeation bysodium chloride.

4. Increased purity of chlorine produced at the anode.

5. Resistance to oxidation.

Those membranes which have both carboxylic and sulfonic groups are muchpreferred embodiments of this invention. They exhibit all of the aboveadvantages and, in addition:

1. Their useful life in operation is surprisingly long.

2. Power consumption in units in which they are employed is surprisinglylow.

The membranes of this invention are prepared from fluorocarbon polymerswith pendant carboxylic acid groups, or pendant carboxylic acid andsulfonic acid groups. The pendant groups may be chemically bonded to thefluorocarbon polymer. Alternatively, they may become integral with thepolymer by impregnation and coating techniques followed bypolymerization as described more fully hereinafter. Since membranesprepared by both procedures are functional equivalents, they will bedescribed in the specification and claims as fluorocarbon polymermembranes having pendant carboxyl groups, or pendant carboxyl andsulfonic groups. Often the polymers will be crosslinked to increaseresistance to solvent and temperature under electrolysis conditions.Many linear polymers, however, are completely satisfactory. For example,a crosslinking agent such as divinyl benzene may be added to a monomermixture used to impregnate or coat a fluorocarbon membrane. Uponcompletion of polymerization, the dimensional stability of the membranemay be greatly improved.

As mentioned above, the carboxylic acid groups may be bonded chemicallyto the fluorocarbon polymer. Alternatively, a polymer having carboxylicacid groups may be combined physically with the fluorocarbon polymer. Inthe latter case, the polymer having carboxylic acid groups may bedispersed uniformly throughout the fluorocarbon matrix or it may bepresent in layers on the fluorocarbon polymer. Such products may beprepared by coating or impregnating a fluorocarbon polymer membranewhich may or may not have sulfonic acid groups with a carboxylic acidgroup-containing monomer and, thereafter, effecting polymerization.

As mentioned above, when both sulfonic acid and carboxylic acid groupsare present on the membrane, it will have high electric conductivity,with a resulting decrease in power cost and increasing commercialadvantages. The advantages are especially apparent when the ratio ofcarboxylic acid groups to sulfonic acid groups is from 1:100 to 100:1.

When practicing this invention, the acid groups may be either in theform of free acid or metal salts.

The cation exchange membrane of this invention wherein the acid groupsare directly attached to the base fluorocarbon polymer may be preparedas follows:

1. A membrane made from a polymer produced by polymerizing a vinyl etherof the general formula:

    CF.sub.2 ═CF--O--(CF.sub.2).sub.n --X

(wherein n is an integer of 2 to 12, preferably 2 to 4; and X is --CH,--COF, --COOH, --COOR, --COOM or --CONR₂ R₃, where R is an alkyl groupcontaining 1 to 10, preferably 1 to 3, carbon atoms; R₂ and R₃ areindividually hydrogen or one of the groups represented by R; and M issodium, potassium or cesium); with tetrafluoroethylene and/or CF₂═CF--O--R_(f) (wherein R_(f) is a perfluorinated alkyl group containing1 to 3 carbon atoms) and hydrolyzing the polymer where necessary to formacid groups;

2. A polymer membrane made by polymerizing a perfluoroacrylic acidrepresented by the general formula:

    CF.sub.2 ═CFCOZ

(wherein Z is fluorine or an alkoxy group containing 1 to 10, preferably1 to 3 carbon atoms, amino or hydroxy group) or a perfluoroacrylicfluoride; with tetrafluoroethylene and CF₂ ═CF--O--R_(f) and hydrolyzingwhere necessary to form acid groups; and

3. A membrane made by polymerizing a perfluorocarbon vinyl ether of thegeneral formula:

    CF.sub.2 ═CF--O--(CF.sub.2).sub.n --X,

a perfluorocarbon sulfonyl fluoride of the general formula:

    FSO.sub.2 CFR.sub.g CF.sub.2 O(CFYCF.sub.2 O).sub.m CF═CF.sub.2

(wherein R_(g) is fluorine or a perfluoroalkyl group having 1 to 10carbon atoms; Y is fluorine or a trifluoromethyl group; and m is aninteger of 1 to 3); with tetrafluoroethylene and/or CF₂ ═CF--O--R_(f) ;and hydrolyzing where necessary to form acid groups.

Cation exchange membranes of the invention in which the pendant acidgroups are physically combined with the base are prepared as follows:

4. A membrane of fluorocarbon polymer membrane, e.g. a homo- orcopolymer of such monomer as tetrafluoroethylene, hexafluoropropylene orperfluorovinyl ether is coated or impregnated with CF₂ ═CF--O--(CF₂)_(n)--X, wherein X has the same meaning as in (1) above, polymerizing and,if necessary, hydrolyzing to form the acid;

5. A copolymer membrane made from a perfluorovinyl ether derived monomerwhich has an LSO₂ group convertible to sulfonic acid of the generalformula:

    LSO.sub.2 CFR.sub.g CF.sub.2 O(CFYCF.sub.2 O).sub.m CF═CF.sub.2

(wherein L is OH, fluorine or OA, where A is a quaternary ammoniumradical), tetrafluoroethylene and CF₂ ═CF--O--R_(f) is impregnated orcoated with CF₂ ═CF--O--(CF₂)_(n) --X, followed by polymerization, and,if necessary, hydrolyzing to form the acid;

6. A membrane made from a perfluorovinyl ether derived monomer having agroup convertible to sulfonic acid group of the general formula LSO₂CFR_(g) CF₂ O(CFYCF₂ O)_(m) CF═CF₂, as in (5) above, withperfluoroacrylic acid or perfluorocarbonyl fluoride, followed bypolymerization, and, if

necessary, hydrolyzing to form the acid;

7. A fluorocarbon polymer membrane having no ion exchange group isimpregnated or coated with a vinyl compound having a COOR group, whereinR is alkyl containing from 1 to 10 carbon atoms, followed bypolymerization, and, if necessary, hydrolyzing to form the acid; and

8. A membrane made from a perfluorovinyl ether derived monomer of thegeneral formula LSO₂ CFR_(g) CF₂ O(CFYCF₂ O)_(m) CF═CF₂ as in (5) above,with a vinyl compound having a COOR group, as in (7) above, followed bypolymerization, and, if necessary, hydrolysis to form the acid.

Among the polymers mentioned in the above, copolymers comprising CF₂═CF--OR_(f) and CF₂ ═CF--O--(CF₂)_(n) --X or CF₂ ═CF--COZ and thecopolymer comprising CF₂ ═CF--OR_(f), CF₂ ═CF₂ and CF₂ ═CF--O--(CF₂)_(n)--X or CF₂ ═CF--COZ are preferred because of the ease with which theycan be formed into membranes.

When the monomers are impregnated into or coated on the polymer in thepreparation of the above membranes, the polymerization may be effectedin the presence of a crosslinking agent or a solvent, if desired.

Typical examples of a fluorinated perfluorovinyl ether of the generalformula:

    CF.sub.2 ═CF--O--(CF.sub.2).sub.n --X

are methyl perfluoro-6-oxa-7-octenoate, methylperfluoro-5-oxa-6-heptenoate, perfluoro-6-oxa-7-ocetanoyl fluoride andperfluoro-6-oxa-7-ocetene nitrile.

Typical examples of a LSO₂ group containing perfluorovinyl etherderivative of the general formula:

    LSO.sub.2 CFR.sub.g CF.sub.2 O(CFYCF.sub.2 O).sub.m CF═CF.sub.2

are triethylammonium salts of perfluoro[2-(2-fluorosulfonylethoxy)-propylvinyl ether], (C₂ H₅)₃ HN--O--SO₂ CF₂CF₂ OCF(CF₃)--CF₂ OCF═CF₂.

Typical examples of the vinyl ether of the general formula:

    CF.sub.2 ═CFOR

are perfluoromethyl perfluorovinyl ethers.

Typical examples of the perfluorocarbon polymer free from COOR groupsare homopolymers of tetrafluoroethylene, hexafluoropropene, vinylidenefluoride, perfluoromethyl perfluorovinyl ether, chlorotrifluoroethylene,1,1,3,3,3,-pentafluoropropene and 1,2,3,3,3-pentafluoropropene,alternating copolymers of these monomers and copolymers of thesemonomers with ethylene.

As crosslinking agents, there may be used fluorinated diolefins of thegeneral formula:

    CF.sub.2 ═CF--O--(CF.sub.2 CF.sub.2 --O).sub.n CF═CF.sub.2,

in addition to such diolefin compounds as, for example, divinylbenzeneand butadiene. When a membrane made from a fluorocarbon polymer withpendant sulfonic groups is coated or impregnated with a monomer such asacrylic acid and polymerized in the presence of divinylbenzene, theresulting cation exchange membrane is greatly improved in dimensionalstability.

As is clear from the above explanation, the cation exchange membranes ofthis invention can be prepared by a variety of methods utilizing manydifferent monomers. They may be homopolymers or copolymers, includingmore than two monomeric units. As is standard in the art, fluorocarbonrefers to fluorine containing monomers which may contain hydrogen,chlorine or other atoms attached to carbon atoms, e.g.chlorotrifluoroethylene and vinylidene fluoride. Perfluorocarbons aremonomers in which the hydrogens are all replaced with fluorine. Forstability, the latter are preferred.

Standard polymerization procedures including solution, emulsion andsuspension polymerization may be employed. Polymerization may beinitiated by free radical mechanisms or other procedures. The resultingpolymer is molded into a membrane according to an ordinary moldingprocedure such as melt fabrication or the like. The cation exchangemembranes may often be prepared directly by casting polymerizationtechniques. When a fluorocarbon polymer having ion exchange groups isimpregnated or coated with acrylic acid or the like monomer havingcarboxylic groups, and, if desired, with a crosslinking agent and thenpolymerized, the polymerization may be in the presence of a free radicalpolymerization catalyst such as a peroxide, by the action of high energyionizing radiation, or by other means.

Generally, the cation exchange membranes used in this invention willhave an exchange capacity, in terms of carboxylic acid groups, of 0.1 to10 milli-equivalents, preferably 0.5 to 4.0 milli-equivalents, per gramof dry resin. When sulfonic acid groups are also present in themembranes, the exchange capacity of the sulfonic acid groups is 0.1 to10 milli-equivalents, preferably 0.5 to 4.0 milli-equivalents, per gramof dry resin.

The cation exchange membrane used in the present invention may sometimesbe reinforced in mechanical strength by incorporating into the membranea net of fibers of other fluorocarbon polymer. For industrial purposes,the use of a cation exchange membrane, which has been linked with Teflonfibers, is preferable, in general. The thickness of the membrane is 0.01to 1.5 mm, preferably 0.05 to 1.5 mm, and may be suitably selected sothat the specific conductivity and current efficiency of the membrane issuch that it may be successfully employed in the electrolysis of sodiumchloride in the selected electrolytic cell.

The cation exchange membranes of this invention contain 5 to 50%, basedon their own weight, of water (sodium type or form membrane). Themembranes are utilized to divide the electrolytic cell employed into acathode chamber and an anode chamber. Electrolysis is performed bycharging the anode chamber with an aqueous sodium chloride solution,while adding to the cathode chamber water, or a dilute sodium hydroxidesolution, which may be recycled to control the concentration of sodiumhydroxide at the outlet of the cathode chamber. The concentration of thesodium chloride solution charged to the anode chamber is normally high,preferably near saturation.

The electrolysis may be effected at a temperature of 0° to 150° C., andheat generated due to the electrolysis is removed by cooling a part ofthe anolyte or catholyte.

In the cathode and anode chambers, there are generated hydrogen andchlorine, respectively. To prevent the membrane from contacting eitherelectrode, a spacer may be interposed between the two. The separation ofthe gases from the liquids is desirably conducted by providing a freespace at the upper portion of each chamber of the electrolytic cell. Inthis case, it is sometimes desirable that the gases and the effluents bedischarged separately, though discharging them together may be effectedin the cathode or anode chambers. When separation of gas from liquid iseffected in the upper free space within the electric cell, the recycleof the electrolyte in each chamber can advantageously be promoted by theascending action of the formed gases, in general. This is particularlyapparent where the electrolytic cell has been so designed that theformed gases are introduced at the back side of each electrode and areascending so that the space between the electrode and the membranesurface is gas free. Amongst the advantages of this design are decreasedpotential depression and the power consumption.

The perpass electrolysis ratio of sodium chloride charged to the anodechamber may be 3 to 50%. This varies depending on the current densityand the manner of heat removal, but is normally maintained as high aspossible.

The liquid in each chamber is desirably stirred by means of the gasesgenerated in the cathode and anode chambers, in addition to the flow ofexternally supplied fluids. For, this purpose also, it is desirable thatan electrode having many vacant spaces such as a metal mesh electrode isused so that the liquid in each chamber can be moved, circulated andstirred with ascending flow of the gases.

As the cathode, the use of an iron electrode which has been plated withnickel or a nickel compound is preferable, in general, from thestandpoint of overpotential. As the anode, the use of a metal mesh orrod electrode which has been coated with an oxide of a noble metal suchas ruthenium or the like is preferred. Use of these types of electrodesmakes it possible to minimize the interval between membranes andelectrodes so that power consumption and potential depression duringelectrolysis are minimized. By the use of the membranes, back-migrationof OH ions is inhibited and the cathode and anode chambers aredistinctly separated from each other. Accordingly, when metal electrodeshigh in mechanical dimensional precision are used in combination withthe cation exchange membrane of this invention, the interval betweeneach electrode and the membrane can be made extremely small, e.g. about1 to 3 mm, so that electrolysis can be effected at a high currentdensity while minimizing the potential depression and while maintaininglow power consumption. These characteristics are not observed in theconventional diaphragm process.

Cation exchange membranes of this invention are resistant to chlorinegenerated in the anode chamber, so that the electrolysis operation canbe carried out stably over a long period of time. Additionally, asindicated above, back-migration of hydroxyl ions is inhibited. As aresult, the pH of the liquid in the anode chamber can be easilymaintained at from neutral to slightly acidic, and thus the content ofoxygen in the chlorine generated in the anode chamber can be maintainedas low as less than 500 p.p.m.

By utilizing the cation exchange membranes of the invention, currentefficiency is far higher than can be achieved with cation exchangemembranes prepared from perfluorocarbon polymers with sulfonic acidgroups as the only ion exchange groups. The production, in the cathodechamber, of sodium hydroxide at a concentration of more than 20% can beeffected with a current efficiency of at least 80%, and about 90 to 98%under optimum conditions. Since the current efficiency is high and thepower consumption low, cells using membranes of this invention can beoperated economically at current densities as high as 20 to 70 A/dm². Aprincipal reason contributing to the high current efficiency is theinhibition of back-migration of OH ions.

The aqueous sodium chloride solution charged to the anode chamber ispurified, as in conventional sodium chloride electrolysis processes. Itmay be subjected to the dechlorination, dissolution and saturation ofsodium chloride, precipitation and separation of magnesium, calcium,iron, etc., and neutralization, as in other procedures. It may also bedesirable to further purify the feed sodium chloride solution with agranular ion exchange resin, particularly a chelate resin, to reduce thecalcium content thereof to an acceptable limit, preferably to less than1 p.p.m.

While this invention should not be limited by theory, it appearspossible that the advantages of this invention are attained because ofthe low dissociation constant of carboxylic acid groups. As a result ofthe low dissociation constant, the carboxyl groups in the membrane incontact with the anolyte having a high hydrogen ion concentration existin the hydrogen form, which makes the membrane structure more compactand effectively inhibits the back-migration of hydroxyl ions. Thiseffect cannot be achieved with membranes in which the only pendantgroups are sulfonic acid groups.

The following non-limiting examples are given by way of illustrationonly.

EXAMPLE 1

A copolymer of perfluoro[2-(2-fluorosulfonylethoxy)propylvinyl ether]with tetrafluoroethylene was molded according to a conventionalpolymer-molding process into a membrane 0.12 mm in thickness. Themembrane was hydrolyzed to prepare a perfluorosulfonic acid type cationexchange membrane having an exchange capacity of 0.91 milli-equivalentsper gram of dry resin. This cation exchange membrane was heated at 100°C. for 3 hours in a solution containing 15% acrylic acid, 15%divinylbenzene, 55% styrene and 0.01% of benzoyl peroxide to impregnatethe membrane with the monomer mixture, which was than polymerized at110° C.

The thus obtained cation exchange membrane, which was a polymer mixturecomprising a perfluorosulfonic acid type polymer and a crosslinkedacrylic acid polymer, contained about 0.81 milli-equivalent/gram dryresin of exchange groups in terms of sulfonic acid groups and 0.23milli-equivalent/gram dry resin of carboxylic acid groups. The thicknessof the cation-exchange membrane was 0.14 mm.

This cation exchange membrane, which had an effective area of 100 dm²,was used to divide an electrolytic cell into a cathode chamber and ananode chamber. 50 Units of such electrolytic cell were arranged inseries so that the respective adjacent electrodes formed a bipolarsystem comprising 50 electrolytic cells.

Using the thus prepared electrolytic cell assembly, electrolysis wasconducted by charging 305 g/l of an aqueous sodium chloride solution toeach cell through the inlet of the anode chamber, and an aqueous sodiumhydroxide solution was recycled while being controlled at aconcentration of 20% by adding water to the outlet of the cathodechamber. The electrolysis was carried out while applying in series acurrent of 5,000 amperes to the chambers.

In this case, the amount of the solution charged to the anode chamberwas controlled to 11,515 kl/hr, the amount of the water added to theoutlet of the cathode chamber was controlled to 1.063 kg/hr, and theaqueous sodium hydroxide solution at the outlet of the cathode chamberwas recycled. As the result, the amount of chlorine generated in theanode chamber was 314.5 kg/hr, the amount of 20% sodium hydroxiderecovered from the cathode chamber was 1,521.8 kg/hr, and the amount ofhydrogen generated from the cathode chamber was 9,325 g/hr. The currentefficiency of the sodium hydroxide recovered from the outlet of thecathode chamber was 95.1%.

COMPARISON EXAMPLE 1

A copolymer of perfluoro[2-(2-fluorosulfonylethoxy)propylvinyl ether]with tetrafluoroethylene was molded into a membrane 0.12 mm inthickness, which was then hydrolyzed to prepare a cation exchangemembrane containing 0.90 milliequivalent/gram dry resin of sulfonic acidgroups.

This membrane was utilized in the same manner as in Example 1, but thecurrent efficiency while producing sodium hydroxide of 35.1%concentration was only 55.7%, and the amount of NaCl in NaOH was 2,000p.p.m. Further, the specific electric conductivity of the membrane was11.3 mho/cm s measured in a 0.1 N aqueous NaOH solution at 25° C.

The specific electric conductivity of the membrane was measured in thefollowing manner:

The membrane was completely brought into --SO₃ Na form and thenequilibrated by dipping at normal temperature for 10 hours in a 0.1 Naqueous NaOH solution which is supplied continuously. Subsequently, themembrane was measured in electric resistivity in the solution byapplying an alternating current of 1,000 cycles, while maintaining thesolution at 25° C., and the specific electric conductivity wascalculated from the thickness and the effective area of the membrane.

COMPARISON EXAMPLE 2

The same copolymer as in Comparison Example 1 was molded into a membrane0.12 mm in thickness, and then hydrolyzed to prepare a cation exchangemembrane containing 0.65 milli-equivlent/gram dry resin of sulfonic acidgroups.

Using this membrane, electrolysis was conducted in the same manner as inExample 1, but the current efficiency while producing sodium hydroxideof 35.1% concentration was only 73%. The specific electric conductivityof the membrane was 4.5 mho/cm as measured in a 0.1 N aqueous NaOHsolution at 25° C.

COMPARISON EXAMPLE 3

A permionic membrane was prepared from a copolymer oftetrafluoroethylene with trifluoroethylene sulfonic acid (CF₂ CFSO₃ H)having pendant sulfonic acid groups, and was reinforced with a Teflonfiber. The thus prepared membrane contained 0.83 milli-equivalent-gramdry resin of sulfonic acid groups, had a thickness of 0.076 mm, andshowed a porosity of 8.7 ml/min.m² under 43 cm H₂ O head.

Using this membrane, electrolysis was conducted in the same manner as inExample 1. The current efficiency for producing sodium hydroxide of34.8% concentration was only 54.3%, and the amount of NaCl in NaOH was3,000 p.p.m.

EXAMPLE 2

The membrane of Comparison Example 3 was impregnated with a 4:1 mixtureof water and a solution containing 15% of acrylic acid, 30% ofdivinylbenzene (purity 55%) and 0.01% of benzoyl peroxide, using asemulsifiers Nonionic NS 230 and Tracks N 700B produced by Nihon YushiCo., to absorb the monomer mixture into the membrane, and the monomerspolymerized at 110° C. The thus treated membrane contained as exchangegroups 0.80 milli-equivalent/gram dry resin of sulfonic acid groups and0.08 milli-equivalent/gram dry resin of carboxylic acid groups.

Using this membrane, electrolysis was conducted in the same manner as inExample 1. The current efficiency for producing sodium hydroxide of34.8% concentration was 96.4%. The amount of NaCl in NaOH was only 80p.p.m.

EXAMPLE 3

A cation exchange membrane was prepared by molding a ternary polymercomprising perfluoro[2-(2-fluorosulfonylethoxy)propylvinyl ether], atetrafluoroethylene and methyl perfluoro-6-oxa-7-octenoate into amembrane reinforced with a reticular material composed of Teflon,followed by hydrolysis.

This cation exchange membrane had 0.71 milli-equivalent/gram dry resinof sulfonic acid groups and 1.5 milli-equivalent/gram dry resin ofcarboxylic acid groups.

Using 50 sheets of this cation exchange membrane which had an effectivearea of 100 dm², electrolysis was conducted in the same manner and byuse of the same electrolytic cell assembly as in Example 1, except that305 g/l of an aqueous sodium chloride solution was recycled in the anodechamber at a rate of 12,820 kg/hr, and water was continuously poured tothe exit solution of the cathode chamber so that the concentration ofsodium hydroxide in said exit solution was maintained at 31.1%. Theamount of the water was controlled to 767.65 kg/hr, and the electrolysiswas carried out while flowing in series a current of 5,000 amperes to 50units of the electrolytic cell. The amount of chlorine generated in theanode chamber was 311.2 kg/hr, the amount of 31.1% sodium hydroxidesolution recovered from the cathode chamber was 1,127.4 kg/hr, and theamount of hydrogen recovered from the cathode chamber was 9,325 g/hr.The current efficiency was 94%.

EXAMPLE 4

A copolymer of perfluoro[2-(2-fluorosulfonylethoxy)propylvinyl ether]with tetrafluoroethylene was molded into a membrane 0.12 mm inthickness, which was then hydrolyzed to prepare a cation exchangemembrane having an exchange capacity, in terms of sulfonic acid groupsof 0.88 milli-equivalent/gram dry resin.

This perfluorosulfonic acid type cation exchange membrane wasimpregnated with a solution of perfluoroacrylic acid, and was thenpolymerized to obtain a perfluorovinyl ether type cation exchangemembrane, in which perfluoroacrylic acid units were present in admixturewith perfluorosulfonic acid units.

This cation exchange membranes contained 0.75 milli-equivalent/gram dryresin of sulfonic acid groups, and 1.1 milli-equivalent/gram dry resinof carboxylic acid groups.

Using 50 sheets of the thus obtained cation exchange membrane which hadan effective area of 100 dm², electrolysis was conducted in the samemanner as in Example 1, except that the concentration of sodiumhydroxide in the exit solution of the cathode chamber was maintained at35.5%. Current efficiency was 95.8%.

EXAMPLE 5

A polymer prepared by the copolymerization of methylperfluoro-6-oxa-7-octenoate, perfluoromethyl perfluorovinyl ether andtetrafluoroethylene was compression molded to form a membrane 0.12 mm inthickness.

This membrane was hydrolyzed to obtain a carboxylic acid typecation-exchange resin membrane having an exchange capacity of 2.1milli-equivalent/gram dry resin.

Using 50 sheets of the thus obtained cation exchange membrane which hadan effective area of 100 dm², electrolysis was conducted in the samemanner and in the same apparatus as in Example 1, at a series current of5,000 amperes to 50 units of electrolytic cells, except that theconcentration of sodium hydroxide in the exit solution of the cathodechamber was maintained at 38%. The current efficiency was 91.6%.

EXAMPLE 6

A copolymer of tetrafluoroethylene with perfluorovinyl ether was moldedinto a membrane of 0.1 mm in thickness. The membrane was impregnatedwith methyl perfluoro-5-oxa-6-heptenoate, polymerized and thenhydrolyzed to prepare a cation exchange membrane having an exchangecapacity, in terms of carboxylic acid groups, of 2.31milli-equivalent/gram dry resin.

Using this cation exchange resin, electrolysis was conducted in the samemanner as in Example 1. The current efficiency for producing sodiumhydroxide of 27% concentration was 98.2%.

EXAMPLE 7

The membrane of Comparison Example 1 was impregnated withmethyl-perfluoro-5-oxa-6-heptenoate, which was then polymerized and washydrolyzed to prepare a cation exchange membrne having an exchangecapacity, in terms of sulfonic acid groups, of 0.77milli-equivalent/gram dry resin, and an exchange capacity, in terms ofcarboxylic acid groups, of 0.42 milli-equivalent/gram dry resin.

Using this cation exchange membrane, electrolysis was conducted in thesame manner as in Example 1. The current efficiency for producing sodiumhydroxide of 35.0% concentration was 96.2%. The amount of NaCl in NaOHwas only 150 p.p.m. Further, the specific electric conductivity of themembrane was 13.2 mho/cm, and the cell voltage did not differ from thatin Comparison Example 2.

EXAMPLE 8

A 2,3-dichloro-perfluorobutane solution of a ternary copolymer of CF₂═CFO(CF₂)_(4l) COOCH₃, CF₂ ═CFOCF₃ and tertrafluoroethylene was coatedon one side of the membrane of Comparison Example 1. After evaporatingthe solvent, the membrane was hot-pressed and then hydrolyzed to preparea cation exchange membrane having a coating of 0.01 mm in thickness. Thethus prepared cation exchange membrane contained 0.83milli-equivalent/gram dry resin of sulfonic acid groups and 0.05milli-equivalent/gram dry resin of carboxylic acid groups.

Using this membrane, electrolysis was conducted in the same manner as inExample 1, while facing the coated side of the membrane to the cathode.The current efficiency for producing sodium hydroxide of 35.5%concentration was 97.1%.

EXAMPLE 9

A ternary copolymer comprising methyl perfluoroacrylate,tetrafluoroethylene and perfluoropropyl vinyl ether was molded into amembrane 0.12 mm in thickness, which was then hydrolyzed to prepare acation exchange membrane containing 1.15 milli-equivlent/gram dry resinof carboxylic acid groups.

Using 50 sheets of this cation exchange membrane which had an effectivearea of 100 dm², electrolysis was conducted in the same manner and byuse of the same apparatus as in Example 1, while flowing in series acurrent of 5,000 amperes to the 50 units of electrolytic cells. Thecurrent efficiency for producing sodium hydroxide of 31.7% concentrationwas 97.2%.

EXAMPLE 10

A solution containing 15% of acrylic acid, 30% of divinylbenzene (purity55%), 55% of styrene and 0.01% of benzoyl peroxide was coated on oneside of the membrane of Comparison Example 1, and was then polymerizedat 110° C. to form a coating 0.005 mm in thickness. The thus coatedcation exchange membrane contained 0.88 milli-equivalent/gram dry resinof sulfonic acid groups and 0.06 milli-equivalent/gram dry resin ofcarboxylic acid groups.

Using this cation exchange membrane, electrolysis was conducted in thesame manner as in Example 1, while facing the coated side of themembrane to the cathode. The current efficiency for producing sodiumhydroxide of 35.1% concentration was 94.8%, and the specific electricconductivity of the membrane was 10.2 mho/cm.

EXAMPLE 11

A copolymer of CF₂ ═CF--O(CF₂)₄ COONa with tetrafluoroethylene wasmolded into a membrane of 0.12 mm in thickness to prepare a cationexchange membrane containing 1.33 milli-equivalent/gram dry resin ofcarboxylic acid groups. using this cation exchange membrane,electrolysis was conducted in the same manner as in Example 1. Thecurrent efficiency for producing sodium hydroxide of 35.8% concentrationwas 92.9%.

EXAMPLE 12

A ternary copolymer comprisingperfluoro[2-(2-fluorosulfonylethoxy)-propylvinyl ether],tetrafluoroethylene and perfluoro-6-oxa-7-obtenoyl fluoride was moldedinto a membrane of 0.12 mm in thickness, which was then hydrolyzed toprepare a cation exchange membrane containing 0.43 milli-equivalent/gramdry resin of sulfonic acid groups and 0.70 milli-equivalent/gram dryresin of carboxylic acid groups.

Using 50 sheets of this cation exchange membrane which had an effectivearea of 100 dm², electrolysis was conducted in the same manner and byuse of the same apparatus as in Example 1, while flowing in series acurrent of 5,000 amperes to the 50 units of electrolytic cell. Thecurrent efficiency for producing sodium hydroxide of 35.6% concentrationwas 98.8%, and the specific electric conductivity of the membrane was9.0 mho/cm.

EXAMPLE 13

A ternary copolymer comprising perfluorovinyl ether, tetrafluoroethyleneand perfluoro-5-oxa-6-heptenoyl fluoride was molded into a membrane 0.12mm in thickness, which was then hydrolyzed to prepare a cation exchangemembrane containing 1.36 milli-equivalent/gram dry resin of carboxylicacid groups.

Using this cation-exchange membrane, electrolysis was conducted in thesame manner as in Example 1. The current efficiency for producing sodiumhydroxide of 35.5% concentration was 93.3%, and the specific electricconductivity of the membrane was 7.2 mho/cm.

EXAMPLE 14

A copolymer of perfluoroacrylic acid with tetrafluoroethylene was moldedinto a membrane 0.12 mm in thickness. This membrane contained 1.88milli-equivalent/gram dry resin of carboxylic acid groups.

Using this membrane, electrolysis was conducted in the same manner as inExample 1. The current efficiency for producing sodium hydroxide of32.5% concentration was 93.6%.

EXAMPLE 15

A quaternary copolymer comprisingperfluoro[2-(2-fluorosulfonylethoxy)-propylvinyl ether],tetrafluoroethylene, perfluoro-5-oxa-6-heptenoyl fluoride andperfluoropropyl vinyl ether was molded into a membrane 0.12 mm inthickness. After reinforcing with a Teflon fiber, the membrane washydrolyzed to prepare a cation-exchange resin membrane containing 0.84milli-equivalent/gram dry resin of sulfonic acid groups and 1.20milli-equivalent/gram dry resin of carboxylic acid groups.

Using this cation-exchange membrane, electrolysis was conducted in thesame manner as in Example 1. The current efficiency for producing sodiumhydroxide of 36.0% concentration was 98.4%.

EXAMPLE 16

The membrane of Comparison Example 1 was impregnated with methylperfluoroacrylate and CF₂ ═CFOCF₂ CF₂ OCF═CF₂, followed bypolymerization and hydrolysis to prepare a cation exchange resinmembrane having an exchange capacity, in terms of carboxylic acidgroups, of 0.79 milli-equivalent/gram dry resin, and, in terms ofsulfonic acid groups, of 0.81 milli-equivalent/gram dry resin.

Using this cation exchange membrane, electrolysis was conducted in thesame manner as in Example 1. The current efficiency for producing sodiumhydroxide of 34.5% concentration was 95.6%.

COMPARISON EXAMPLE 4

A hydrocarbon base cation-exchange membrane having an exchange capacityof 1.8 milli-equivalents/gram of dry resin of carboxylic acid groups and0.8 milli-equivalents/gram of dry resin of sulfonic acid groups wasprepared by sulfonating a membrane prepared from an acrylicacid-styrene-divinylbenzene copolymer. The membrane was converted to thesodium form and installed in an electrolyzer under the followingconditions:

Effective size of membrane--15 cm² (5×3 cm)

Current density--50 A/dm²

Anode--Nobel metal coated

Concentration of NaOH--35% at 90° C.

Cathode--Iron plate

Catholyte--Aq. NaCl, 3 N, pH 3

The membrane fractured after 112 hours of operation. On the other hand,the membrane of Example 12 functioned effectively for over one year.

COMPARISON EXAMPLE 5

A membrane similar to that of the previous comparison example wasprepared in which the exchange capacity was 1.9 milli-equivalents/gramdry resin of carboxylic acid groups and 2.0 milli-equivalents/gram ofdry resin of sulfonic acid groups. It was similarly employed in anelectrolyzer under the same conditions.

After only 48 hours of operation, the membrane was discolored and hadseveral cracks. No such cracks or discoloration were observed in themembrane of Example 12.

What is claimed is:
 1. A process for the electrolysis of an aqueoussodium chloride solution which comprises passing an electric currentthrough said solution in an electrolytic cell separated into an anodechamber and a cathode chamber by a cation exchange membrane consistingessentially of a fluorocarbon polymer containing pendant carboxylic acidand sulfonic acid groups, the total ion exchange capacity of sulfonicand carboxylic acid groups being from 0.5 to 4.0 milliequivalents pergram of dry resin, the ratio of carboxylic acid groups to sulfonic acidgroups being in the range of from 1:100 to 100:1.
 2. The process ofclaim 1, wherein the ion exchange capacity of the carboxylic acid groupsin said membrane is at least 0.005 milliequivalent per gram of dryresin.
 3. The process of claim 1, wherein a fluorocarbon polymercontaining pendant carboxylic acid groups is present on the surface ofthe membrane.
 4. The process of claim 1, wherein a fluorocarbon polymercontaining pendant carboxylic acid groups is present on one surface ofthe membrane.
 5. The process of claim 1, wherein the membrane in thesodium salt form has a water content of from 5 to 50% by weight.
 6. Theprocess of claim 1, wherein said membrane comprises a combination of afluorocarbon polymer having pendant carboxylic acid groups with afluorocarbon polymer having pendant sulfonic acid groups.
 7. The processof claim 1, wherein said fluorocarbon polymer comprises aperfluorocarbon polymer having carboxylic acid groups and sulfonic acidgroups.
 8. The process of claim 1, wherein the cation exchange membraneis fiber reinforced.
 9. The process of claim 1, wherein saidfluorocarbon polymer comprises a copolymer of at least one oftetrafluoroethylene and CF₂ ═CF--O--R_(f) wherein R_(f) is aperfluorinated alkyl group containing 1 to 3 carbon atoms withfluorocarbon vinyl monomers containing carboxylic acid groups andsulfonic acid groups or functional groups which can be converted tocarboxylic acid or sulfonic acid groups.
 10. The process of claim 1,wherein said fluorocarbon polymer comprises a copolymer of afluorocarbon vinyl monomer having the general formula:

    CF.sub.2 =CF--O--(CF.sub.2).sub.n --X

wherein n is an integer of 2 to 12, and X is CN, COF, COOH, COOR, COOMor CONR₂ R₃ where R is an alkyl group containing 1 to 10 carbon atoms,R₂ and R₃ are individually hydrogen or an alkyl group containing 1 to 10carbon atoms, and M is sodium, potassium or cesium; and aperfluorocarbon sulfonyl fluoride having the general formula:

    F--SO.sub.2 --CFR.sub.g --CF.sub.2 --O(CFYCF.sub.2 O).sub.m --CF═CF.sub.2

wherein R_(g) is fluorine or a perfluoroalkyl group having 1 to 10carbon atoms, Y is fluorine or a trifluoromethyl group, and m is aninteger of 1 to 3, with at least one of tetrafluoroethylene and CF₂═CF--O--R_(f) wherein R_(f) is a perfluorinated alkyl group containing 1to 3 carbon atoms, which copolymer is hydrolyzed, if necessary, to formsaid acid groups.
 11. The process of claim 1, wherein said fluorocarbonpolymer comprises a copolymer of a perfluoroacrylic acid having thegeneral formula:

    CF.sub.2 ═CF--COZ

wherein Z is fluorine, an alkoxy group containing 1 to 10 carbon atoms,amino or hydroxy; and a perfluorocarbon sulfonyl fluoride having thegeneral formula:

    F--SO.sub.2 --CFR.sub.g --CF.sub.2 --O(CFYCF.sub.2 O).sub.m --CF═CF.sub.2

wherein R_(g) is fluorine or a perfluoroalkyl group having 1 to 10carbon atoms, Y is fluorine or a trifluoromethyl group, and m is aninteger of 1 to 3, with at least one of tetrafluoroethylene and CF₂═CF--O--R_(f) wherein R_(f) is a perfluorinated alkyl group containing 1to 3 carbon atoms, which copolymer is hydrolyzed, if necessary, to formsaid acid groups.
 12. The process of claim 1, wherein said cationexchange membrane is prepared by impregnating or coating a membranecomprising a copolymer of (A) a perfluorovinyl ether having the generalformula:

    LSO.sub.2 CFR.sub.g CF.sub.2 O(CFYCF.sub.2 O).sub.m --CF═CF.sub.2

wherein L is hydroxy, fluorine or OA, where A is a quaternary ammoniumradical; R_(g) is fluorine or a perfluoroalkyl group having 1 to 10carbon atoms; Y is fluorine or a trifluoroemethyl group; and m is aninteger of 1 to 3, and (B) at least one of tetrafluoroethylene and CF₂═CFOR_(f) wherein R_(f) is a perfluorinated alkyl group containing 1 to3 carbon atoms, with a vinyl compound having carboxylic acid groups orderivatives thereof, polymerizing said impregnated or coated vinylcompound, and hydrolyzing, if necessary, to form said acid groups. 13.The process of claim 12, wherein said vinyl compound is at least onemember selected from the group consisting of compounds having theformula:

    CF.sub.2 ═CF--O--(CF.sub.2).sub.n --X

wherein n is an integer of 2 to 12, and X is CN, COF, COOH, COOR, COOMor CONR₂ R₃, where R is an alkyl group containing 1 to 10 carbon atoms,R₂ and R₃ are individually hydrogen or an alkyl group containing 1 to 10carbon atoms, and M is sodium, potassium or cesium, and compounds havingthe formula:

    CF.sub.2 ═CF--COZ

wherein Z is fluorine or an alkoxy group containing 1 to 10 carbonatoms.
 14. The process of claim 1, wherein said cation exchange membraneis prepared by impregnating or coating a membrane comprising a copolymerof (A) a perfluorovinyl ether having the general formula:

    LSO.sub.2 CFR.sub.g CF.sub.2 O(CFYCF.sub.2 O).sub.m --CF═CF.sub.2

wherein L is hydroxy, fluorine or OA, where A is a quaternary ammoniumradical; R_(g) is fluorine or a perfluoroalkyl group having 1 to 10carbon atoms; Y is fluorine or a trifluoromethyl group; and m is aninteger of 1 to 3, and (B) at least one of tetrafluoroethylene and CF₂═CFOR_(f) wherein R_(f) is a perfluorinated alkyl group containing 1 to3 carbon atoms, with a solution of a polymer having carboxylic acidgroups or derivatives thereof, and hydrolyzing, if necessary, to formsaid acid groups.
 15. A process according to claims 12, 13 or 14,wherein the impregnating or coating is conducted on one surface of themembrane.
 16. The process of claim 15, wherein said carboxylic acidgroups are contained predominantly on said one surface of the membraneand wherein the membrane is disposed in said electrolytic cell such thatsaid surface having the carboxylic acid groups faces the cathode side ofthe cell.
 17. A process according to claims 9, 10, 11, 12, 13 or 14,wherein the cation exchange membrane is fiber reinforced.
 18. Theprocess of claim 1, wherein the thickness of the membrane is 0.05 to 1.5mm.
 19. The process of claim 1, wherein the electrolysis is conducted ata temperature of 0° to 150° C. while charging an aqueous sodium chloridesolution into the anode chamber and adding water or an aqueous dilutesodium hydroxide solution into the cathode chamber to adjust theconcentration of sodium hydroxide to more than 20%.
 20. The process ofclaim 1, wherein the aqueous sodium chloride solution charged to theanode compartment is purified with an ion exchange resin.
 21. Theprocess of claim 20, wherein the aqueous sodium chloride solution has acalcium content of less than 1 ppm.
 22. A cation exchange membranesuitable for use in the electrolysis of an aqueous sodium chloridesolution consisting essentially of a fluorocarbon polymer containingpendant carboxylic acid and sulfonic acid groups, the total ion exchangecapacity of sulfonic and carboxylic acid groups being from 0.5 to 4.0milliequivalents per gram of dry resin, the ratio of carboxylic acidgroups to sulfonic acid groups being in the range of 1:100 to 100:1. 23.A cation exchange membrane in accordance with claim 22, wherein the ionexchange capacity of the carboxylic acid groups in said membrane is atleast 0.005 milliequivalent per gram of dry resin.
 24. A cation exchangemembrane in accordance with claim 22, wherein a fluorocarbon polymercontaining pendant carboxylic acid groups is present on the surface ofthe membrane.
 25. A cation exchange membrane in accordance with claim22, wherein a fluorocarbon polymer containing pendant carboxylic acidgroups is present on one surface of the membrane.
 26. A cation exchangemembrane in accordance with claim 22, which has a water content in thesodium salt form of from 5 to 50% by weight.
 27. A cation exchangemembrane in accordance with claim 22, which comprises a combination of afluorocarbon polymer having pendant carboxylic acid groups with afluorocarbon polymer having pendant sulfonic acid groups.
 28. A cationexchange membrane in accordance with claim 22, wherein said fluorocarbonpolymer comprises a perfluorocarbon polymer having carboxylic acidgroups and sulfonic acid groups.
 29. A cation exchange membrane inaccordance with claim 22, which includes a fiber reinforcement.
 30. Acation exchange membrane in accordance with claim 22, wherein saidfluorocarbon polymer comprises a copolymer of at least one oftetrafluoroethylene and CF₂ ═CF--O--R_(f) wherein R_(f) is aperfluorinated alkyl group containing 1 to 3 carbon atoms withfluorocarbon vinyl monomers containing carboxylic acid groups andsulfonic acid groups or functional groups which can be converted tocarboxylic acid or sulfonic acid groups.
 31. A cation exchange membranein accordance with claim 22, wherein said fluorocarbon polymer comprisesa copolymer of a fluorocarbon vinyl monomer having the general formula:

    CF.sub.2 ═CF--O--(CF.sub.2).sub.n --X

wherein n is an integer of 2 to 12, and X is CN, COF, COOH, COOR, COOMor CONR₂ R₃, where R is an alkyl group containing 1 to 10 carbon atoms,R₂ and R₃ are individually hydrogen or an alkyl group containing 1 to 10carbon atoms, and M is sodium, potassium or cesium; and aperfluorocarbon sulfonyl fluoride having the general formula:

    F--SO.sub.2 --CFR.sub.g --CF.sub.2 --O(CFYCF.sub.2 O).sub.m --CF═CF.sub.2

wherein R_(g) is fluorine or a perfluoroalkyl group having 1 to 10carbon atoms, Y is fluorine or a trifluoromethyl group, and m is aninteger of 1 to 3, with at least one of tetrafluoroethylene and CF₂═CF--O--R_(f) wherein R_(f) is a perfluorinated alkyl group containing 1to 3 carbon atoms, which copolymer is hydrolyzed, if necessary, to formsaid acid groups.
 32. A cation exchange membrane in accordance withclaim 22, wherein said fluorocarbon polymer comprises a copolymer of aperfluoroacrylic acid having the general formula:

    CF.sub.2 ═CF--COZ

wherein Z is fluorine, an alkoxy group containing 1 to 10 carbon atoms,amino or hydroxy; and a perfluorocarbon sulfonyl fluoride having thegeneral formula:

    F--SO.sub.2 --CFR.sub.g --CF.sub.2 --O(CFYCF.sub.2 O).sub.m --CF═CF.sub.2

wherein R_(g) is fluorine or a perfluoroalkyl group having 1 to 10carbon atoms, Y is fluorine or a trifluoromethyl group, and m is aninteger of 1 to 3, with at least one of tetrafluoroethylene and CF₂═CF--O--R_(f) wherein R_(f) is a perfluorinated alkyl group containing 1to 3 carbon atoms, which copolymer is hydrolyzed, if necessary, to formsaid acid groups.
 33. A cation exchange membrane in accordance withclaim 22, wherein said cation exchange membrane is prepared byimpregnating or coating a membrane comprising a copolymer of (A) aperfluorovinyl ether having the general formula:

    LSO.sub.2 CFR.sub.g CF.sub.2 O(CFYCF.sub.2 O).sub.m --CF═CF.sub.2

wherein L is hydroxy, fluorine or OA, where A is a quaternary ammoniumradical; R_(g) is fluorine or a perfluoroalkyl group having 1 to 10carbon atoms; Y is fluorine or a tetrafluoromethyl group; and m is aninteger of 1 to 3, and (B) at least one of tetrafluoroethylene and CF₂═CFOR_(f) wherein R_(f) is a perfluorinated alkyl group containing 1 to3 carbon atoms, with a vinyl compound having carboxylic acid groups orderivatives thereof, polymerizing said impregnated or coated vinylcompound, and hydrolyzing, if necessary, to form said acid groups.
 34. Acation exchange membrane in accordance with claim 33, wherein said vinylcompound is at least one member selected from the group consisting ofcompounds having the formula:

    CF.sub.2 ═CF--O--(CF.sub.2).sub.n --X

wherein n is an integer of 2 to 12, and X is CN, COF, COOH, COOR, COOMor CONR₂ R₃, where R is an alkyl group containing 1 to 10 carbon atoms,R₂ and R₃ are individually hydrogen or an alkyl group containing 1 to 10carbon atoms, and M is sodium, potassium or cesium, and compounds havingthe formula:

    CF.sub.2 ═CF--COZ

wherein Z is fluorine or an alkoxy group containing 1 to 10 carbonatoms.
 35. A cation exchange membrane in accordance with claim 22,wherein said cation exchange membrane is prepared by impregnating orcoating a membrane comprising a copolymer of (A) a perfluorovinyl etherhaving the general formula:

    LSO.sub.2 CFR.sub.g CF.sub.2 O(CFYCF.sub.2 O).sub.m --CF═CF.sub.2

wherein L is hydroxy, fluorine or OA, where A is a quaternary ammoniumradical; R_(g) is fluorine or a perfluoroalkyl group having 1 to 10carbon atoms; Y is fluorine or a trifluoromethyl group; and m is aninteger of 1 to 3, and (B) at least one of tetrafluoroethylene and CF₂═CFOR_(f) wherein R_(f) is a perfluorinated alkyl group containing 1 to3 carbon atoms, with a solution of a polymer having carboxylic acidgroups or derivatives thereof, and hydrolyzing, if necessary, to formsaid acid groups.
 36. A cation exchange membrane in accordance withclaims 33, 34 or 35, wherein the impregnating or coating is conducted onone surface of the membrane.
 37. A cation exchange membrane inaccordance with claim 22, wherein the thickness of the membrane is 0.05to 1.5 mm.
 38. An electrolytic cell comprising an anode chamber and acathode chamber separated by a cation exchange membrane which issuitable for the production of aqueous sodium hydroxide in the cathodechamber wherein an aqueous solution of sodium chloride is charged intothe anode chamber, said membrane consisting essentially of afluorocarbon polymer containing pendant carboxylic acid and sulfonicacid groups, said membrane having a total ion exchange capacity ofsulfonic and carboxylic acid groups of from 0.5 to 4.0 milliequivalentsper gram of dry resin, the ratio of carboxylic acid groups to sulfonicacid groups being in the range of 1:100 to 100:1.
 39. An electrolyticcell in accordance with claim 38, wherein the ion exchange capacity ofthe carboxylic acid groups in said membrane is at least 0.005milliequivalent per gram of dry resin.
 40. An electrolytic cell inaccordance with claim 38, wherein a fluorocarbon polymer containingpendant carboxylic acid groups is present on the surface of themembrane.
 41. An electrolytic cell in accordance with claim 38, whereina fluorocarbon polymer containing pendant carboxylic acid groups ispresent on one surface of the membrane.
 42. An electrolytic cell inaccordance with claim 38, wherein the membrane in the sodium salt formhas a water content of from 5 to 50% by weight.
 43. An electrolytic cellin accordance with claim 38, wherein the membrane comprises acombination of a fluorocarbon polymer having pendant carboxylic acidgroups with a fluorocarbon polymer having pendant sulfonic acid groups.44. An electrolytic cell in accordance with claim 38, wherein saidfluorocarbon polymer comprises a perfluorocarbon polymer havingcarboxylic acid groups and sulfonic acid groups.
 45. An electrolyticcell in accordance with claim 38, wherein the cation exchange membraneis fiber reinforced.
 46. An electrolytic cell in accordance with claim38, wherein said fluorocarbon polymer comprises a copolymer of at leastone of tetrafluoroethylene and CF₂ ═CF--O--R_(f) wherein R_(f) is aperfluorinated alkyl group containing 1 to 3 carbon atoms withfluorocarbon vinyl monomers containing carboxylic acid groups andsulfonic acid groups or functional groups which can be converted tocarboxylic acid or sulfonic acid groups.
 47. An electrolytic cell inaccordance with claim 38, wherein said fluorocarbon polymer comprises acopolymer of a fluorocarbon vinyl monomer having the general formula:

    CF.sub.2 ═CF--O--(CF.sub.2).sub.n --X

wherein n is an integer of 2 to 12, and X is CN, COF, COOH, COOR, COOMor CONR₂ R₃, where R is an alkyl group containing 1 to 10 carbon atoms,R₂ and R₃ are individually hydrogen or an alkyl group containing 1 to 10carbon atoms, and M is sodium, potassium or cesium; and aperfluorocarbon sulfonyl fluoride having the general formula:

    F--SO.sub.2 --CFR.sub.g --CF.sub.2 --O(CFYCF.sub.2 O).sub.m --CF═CF.sub.2

wherein R_(g) is fluorine or a perfluoroalkyl group having 1 to 10carbon atoms, Y is fluorine or a trifluoromethyl group, and m is aninteger of 1 to 3, with at least one of tetrafluoroethylene and CF₂═CF--O--R_(f) wherein R_(f) is a perfluorinated alkyl group containing 1to 3 carbon atoms, which copolymer is hydrolyzed, if necessary, to formsaid acid groups.
 48. An electrolytic cell in accordance with claim 38,wherein said fluorocarbon polymer comprises a copolymer of aperfluoroacrylic acid having the general formula:

    CF.sub.2 ═CF--COZ

wherein Z is fluorine, an alkoxy group containing 1 to 10 carbon atoms,amino or hydroxy; and a perfluorocarbon sulfonyl fluoride having thegeneral formula:

    F--SO.sub.2 --CFR.sub.g --CF.sub.2 --O(CFYCF.sub.2 O).sub.m --CF═CF.sub.2

wherein R_(g) is fluorine or a perfluoroalkyl group having 1 to 10carbon atoms, Y is fluorine or a trifluoromethyl group, and m is aninteger of 1 to 3, with at least one of tetrafluoroethylene and CF₂═CF--O--R_(f) wherein R_(f) is a perfluorinated alkyl group containing 1to 3 carbon atoms, and hydrolyzing, if necessary, to form said acidgroups.
 49. An electrolytic cell in accordance with claim 38, whereinsaid cation exchange membrane is prepared by impregnating or coating amembrane comprising a copolymer of (A) a perfluorovinyl ether having thegeneral formula:

    LSO.sub.2 CFR.sub.g CF.sub.2 O(CFYCF.sub.2 O).sub.m --CF═CF.sub.2

wherein L is hydroxy, fluorine or OA, where A is a quaternary ammoniumradical; R_(g) is fluorine or a perfluoroalkyl group having 1 to 10carbon atoms; Y is fluorine or a trifluoromethyl group; and m is aninteger of 1 to 3, and (B) at least one of tetrafluoroethylene and CF₂═CFOR_(f) wherein R_(f) is a perfluorinated alkyl group containing 1 to3 carbon atoms, with a vinyl compound having carboxylic acid groups orderivatives thereof, polymerizing said impregnated or coated vinylcompound, and hydrolyzing, if necessary, to form said acid groups. 50.An electrolytic cell in accordance with claim 49, wherein said vinylcompound is at least one member selected from the group consisting ofcompounds having the formula:

    CF.sub.2 ═CF--O--(CF.sub.2).sub.n --X

wherein n is an integer of 2 to 12, and X is CN, COF, COOH, COOR, COOMor CONR₂ R₃, where R is an alkyl group containing 1 to 10 carbon atoms,R₂ and R₃ are individually hydrogen or an alkyl group containing 1 to 10carbon atoms, and M is sodium, potassium or cesium, and compounds havingthe formula:

    CF.sub.2 ═CF--COZ

wherein Z is fluorine or an alkoxy group containing 1 to 10 carbonatoms.
 51. An electrolytic cell in accordance with claim 38, whereinsaid cation exchange membrane is prepared by impregnating or coating amembrane comprising a copolymer of (A) a perfluorovinyl ether having thegeneral formula:

    LSO.sub.2 CFR.sub.g CF.sub.2 O(CFYCF.sub.2 O).sub.m --CF═CF.sub.2

wherein L is hydroxy, fluorine or OA, where A is a quaternary ammoniumradical; R_(g) is fluorine or a perfluoroalkyl group having 1 to 10carbon atoms; Y is fluorine or a trifluoromethyl group; and m is aninteger of 1 to 3, and (B) at least one of tetrafluoroethylene and CF₂═CFOR_(f) wherein R_(f) is a perfluorinated alkyl group containing 1 to3 carbon atoms, with a solution of a polymer having carboxylic acidgroups or derivatives thereof, and hydrolyzing, if necessary, to formsaid acid groups.
 52. An electrolytic cell in accordance with claims 49,50 or 51, wherein the impregnating or coating is conducted on onesurface of the membrane.
 53. An electrolytic cell in accordance withclaim 38, wherein said carboxylic acid groups are containedpredominantly on said one surface of the membrane and wherein themembrane is disposed in said electrolytic cell such that said surfacehaving the carboxylic acid groups faces the cathode side of the cell.54. An electrolytic cell in accordance with claim 38, wherein thethickness of the membrane is 0.05 to 1.5 mm.